1. Technical Field
The present invention relates to an hydraulic arrangement for operating a load body on a heavy vehicle in which the load body is connected to the frame of the vehicle by way of at least one articulated joint that enables the load body to tip, and to be moved between a lowered transport position and a raised tipping position.
2. Background
A heavy vehicle such as a dumper truck that, for instance, carries gravel in a load body typically discharges its load by tipping the load body. The load body, which is supported at the rear end of the vehicle, can usually be tipped to a maximum of 60° to 70°. Tipping is performed either by a directly acting lifting cylinder or indirectly via a knuckle joint arrangement that entails a system of linkages coupled to the load body and vehicle frame, and which is actuated by lifting cylinders.
There are many requirements to be taken into account when designing a tipping system. The system should, for example, be efficient; that is to say, it should rapidly tip the load and rapidly return to the home or transport position. It is advantageous to be able to use “small” hydraulic cylinders that have a short stroke and moderate hydraulic pressure, since this means that the hydraulic system can be of a more simple design and does not need to function with large flows of hydraulic fluid.
Some of the requirements, however, conflict with one another. A short stroke, for example, requires the production of large forces, and in turn, big cylinders. In such a configuration, the tipping geometry has great significance in the interplay of resulting forces.
If a point of action used for a tipping cylinder between the vehicle frame and the load body is applied essentially at the greatest distance from the load body bearing points on the frame, the force required of the piston cylinder is relatively low, but the piston cylinder then needs to be long. Use is therefore often made of telescopic cylinders that extend in a number of stages. This again becomes a slow process since the cylinder's large diameter is also offset by large oil flows per unit of time. A telescopic cylinder is moreover mechanically sensitive or vulnerable, since it has large sealing surfaces with increased risk of leakage.
Another example of cylinder mounting is to place single cylinders on either side of the load body; that is to say, with points of action shifted towards the load body bearing point on the vehicle frame. Since this shortens the lever arm, the power required from the tipping cylinders is increased.
U.S. Pat. No. 4,488,756 describes a tipping system having a master cylinder and two secondary cylinders that act in different directions. The master cylinder is fitted at the front edge of the load body and the two secondary cylinders are fitted with their point of action between the two ends of the load body. The master cylinder is intended to act during a first phase of the tipping in which the power requirement is greatest. This solution requires the hydraulic system to become relatively complex.